1. Field of the Invention
The present invention relates to a direct-modulation type frequency modulator utilizing a PLL (Phase-Locked Loop).
2. Description of the Related Art
In the FM broadcasting or in-car audio systems, used is a frequency modulator which generates a frequency-modulated signal based on an audio signal (modulation signal) to be transmitted. For example, in Reference (1) listed in the following Related Art List, disclosed is a direct-modulation type frequency modulator in which a PLL is configured by a voltage-controlled oscillator (hereinafter referred to as VCO), a divider, a phase comparator and a loop filter, and the audio signal being a modulation signal is superimposed with an input signal of the VCO.
As disclosed in Reference (1), in the conventional direct-modulation frequency modulator a method is implemented in which the modulation signal is coupled by using a coupling capacitor or a resistor for voltage addition between the output terminal of the loop filter and the input terminal of the VCO.
Related Art List
(1) Japanese Patent Application Laid-Open No. Hei09-69729.
However, since in the conventional practice the modulation signal is superimposed with the high-frequency signal using a coupling capacitor or a resistor, there is a problem that the impedance of the loop filter in the PLL affects the input signal which is the modulation signal. When the modulation signal is influenced by the loop filter, the high-frequency component of the modulation signal is attenuated. This causes the distortion in the signal, for example in cases where the audio signal is modulated. Also, there are cases where the signal output from the loop filter is leaked in the side of the circuit that generates the modulation signal. Further, since the coupling capacitor is required, the number of circuit components needed for a circuitry is increased.